A major problem in two-stroke engines is the process of purging exhaust gases and, during the same stroke, providing combustion air. The process of purging the exhaust gases is commonly referred to as “scavenging”. Although fuel injection systems mitigate this problem to some extent, proper scavenging is indispensable for achieving high efficiency and low exhaust emissions.
A problem with scavenging in conventional two-stroke engines has been to prevent these two gas masses from mixing, with all the well-known resulting drawbacks regarding fuel efficiency and emissions. This problem was addressed by the prior invention.
In the prior invention, scavenging was achieved by locating at least one and preferably a number of air intake valves in the head of each cylinder, and at least one and preferably a number of exhaust gas discharge openings in the lower cylinder walls. The air intake valves were controlled solely by air pressure differentials, generated by fluctuating pressure inside the cylinder on one side and in the air supply chamber on the other side. When the piston rim cleared the exhaust openings on its downstroke, pressure in the cylinder decreased below the pressure in the air supply chamber, causing the air intake valves to open and allow for the inflow of scavenging air. A scavenging blower was used to force air into the air supply chamber and thence through the valves, in order to more effectively purge the exhaust gases form the cylinder as the piston descended. This arrangement can operate in an internal combustion engine utilizing either the Diesel or Otto processes.
Test results with a prototype engine according to the prior invention are very encouraging. However, it has been realized that notwithstanding excellent results to date, further improvement is possible.
In particular it has been realized that it would be beneficial to assist the passive valve check bodies of the prior invention to close more promptly after the upward-moving piston has closed the exhaust ports. This would increase the crankshaft angle available to compression, and thus facilitate a higher effective compression ratio in the combustion chamber.